Method and apparatus for scalable compression and decompression of a digital motion video signal

ABSTRACT

In a method and apparatus for scalably encoding in real time a sequence of digital video frames, a target digital processor class to be used for decoding the sequence is selected. A stillperiod value is selected in accordance with the selected target digital processor class. Every Nth frame of the sequence is then encoded as a still frame, and all other frames are encoded using interframe difference values, wherein N is equal to the stillperiod value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending U.S. patent application Ser. No. 07/861,227, filed Apr. 1, 1992, now U.S. Pat. No. 5,325,126.

FIELD OF THE INVENTION

This invention relates to video signal processing generally and particularly to systems for providing a compressed digital video signal representative of a full color video signal.

BACKGROUND OF THE INVENTION

In real time video systems, compression and decompression are typically done using the same or similar hardware at roughly the same speed. Real time video systems have often required hardware that is too expensive for a single user, or such systems have sacrificed picture quality in favor of lower cost hardware. This problem has been bypassed by the use of presentation level video systems where the compression is performed on expensive hardware, but the decompression is done by low cost hardware. This solution works only in situations where the single-user system needs only to playback compressed video which has been prepared ahead of time.

It is an object of the present invention to provide a system for compressing and decompressing motion video in real time which may operate on lower cost hardware while maintaining acceptable picture quality.

Further objects and advantages of the invention will become apparent from the description of the invention which follows.

SUMMARY OF THE INVENTION

In a method and apparatus for scalably decoding a sequence of encoded digital video frames on a digital processor, a frame discard interval is determined in accordance with the processing power of the digital processor and a counter is initialized. A frame is selected from the sequence for decoding. The selected frame is decoded and the counter is incremented. If the counter is less than the discard interval then the process is repeated; otherwise, at least one subsequent frame from the sequence is replaced with a sync frame which is thereafter decoded.

A method and apparatus for scalably encoding in real time a sequence of digital video frames. A target digital processor class to be used for decoding the sequence is selected. A stillperiod value is selected in accordance with the selected target digital processor class. Every Nth frame of the sequence is then encoded as a still frame, and all other frames are encoded using interframe difference values, wherein N is equal to the stillperiod value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating the operation of a decoder according to a preferred embodiment of the present invention.

FIG. 2A shows the use of a corresponding previous pixel to perform intra-frame decoding in accordance with a preferred embodiment of the present invention.

FIG. 2B shows the use of a corresponding previous pixel to perform inter-frame decoding in accordance with a preferred embodiment of the present invention.

FIG. 3 is a flow diagram illustrating the operation of an encoder according to a preferred embodiment of the present invention.

FIG. 3A is a flow diagram illustrating the vector quantization and run-length encoding procedures of the encoder of FIG. 3.

FIG. 3B is a flow diagram illustrating the Huffman encoder of FIG. 3.

FIG. 4A is a flow diagram illustrating a video compression system according to the present invention.

FIG. 4B is a flow diagram illustrating a video decompression system according to the present invention.

FIG. 5A is a flow diagram showing the operation of a preferred system for scalably decoding a digital video sequence in accordance with the present invention.

FIG. 5B is a flow diagram showing the operation of an alternative preferred system for scalably decoding a digital video sequence in accordance with the present invention.

FIG. 5C is a flow diagram showing the operation of a preferred system for scalably encoding a digital video sequence in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is shown a flow diagram illustrating the operation of a decoder for decoding a bitstream 100 according to a preferred embodiment of the present invention. Bitstream 100 represents a motion video sequence of one or more images which have been encoded in real time. Encoded data from bitstream 100 is applied to Huffman decoder 110 to derive a code-book index representing the position of a code-word within a lexicographically-ordered list of code-words. The code-book index is then used as an entry point to determine an index value from look-up table 120. Comparing means 130 are provided for comparing the index value to a predetermined threshold. If the index value is greater than the predetermined threshold, then copying means 140 determines at least one current pixel by copying a corresponding previous pixel into the location of a current pixel. In a preferred embodiment, copying means 140 calculates the amount that the index value exceeds the predetermined threshold, and then determines that number of current pixels by copying that number of corresponding previous pixels into respective current pixel locations. If the index value is not greater than the predetermined threshold, then the index value is used as an entry point to determine at least one vector value from vector table 150. Means 160 then determines at least one current pixel from a vector value and a corresponding previous pixel. In the preferred embodiment, means 160 uses the index value to determine two vector values which are adjacent in vector table 150. The two vector values are then used by means 160 to determine two adjacent current pixels from two corresponding previous pixels.

The preferred embodiment of the present invention is intended for compression of 8-bit planes of an image. The present invention may also be used with YUV images, and other image formats, including 12-bit image formats, may also be used. In the preferred embodiment, the same compression and decompression process steps are applied to each color component of each image in a sequence. As used below, the term image refers to a single color component of an image.

In the preferred embodiment, each image in the sequence is encoded as either a still image or by using inter-frame differences. During the encoding of an image, each pixel in the image is subtracted from a corresponding previous pixel and the differences are encoded. As shown in FIG. 2A, if the image (200) is being encoded as a still (intra-frame encoding), the corresponding previous pixel (210) is preferably the pixel directly above the current pixel being encoded (220). As shown in FIG. 2B, if the image (240) is encoded using inter-frame differences, the corresponding previous pixel (232) is preferably the pixel in the previous image (230) located in the same position (in the bitmap) as the current pixel being encoded (234). In either case, there is a difference image (whose values tend to cluster around zero) which is encoded. Difference images are preferably encoded using 2-D vector quantization, with some run-length encoding added to help encode large areas of zeros efficiently.

In the preferred embodiment, bitstream 100 includes the following fields for each frame in a sequence: StillFlag, DataSize, ImageHeight, ImageWidth, Flags, VectorSet, a Huffman table descriptor for the image, and Huffman encoded data for the Y, V, U planes. The StillFlag field indicates whether the image is a still, DataSize indicates the size of the bitstream in bits, and ImageHeight and ImageWidth give the size of the decoded image in pixels. The Flags field indicates whether the data in the bitstream represents an image that has been encoded at full resolution, half vertical resolution, half horizontal resolution, or half vertical and half horizontal resolution. Such half resolution images may be obtained prior to encoding by subsampling the full resolution image in one or both dimensions. In the preferred embodiment, an image for encoding is considered to be at full resolution if it is ImageHeight pixels high and ImageWidth pixels wide; it is considered to be at half vertical resolution if it is ImageHeight pixels high and ImageWidth/2 pixels wide; it is considered to be at half horizontal resolution if it is ImageHeight/2 pixels high and ImageWidth pixels wide; and it is considered to be at half vertical and half horizontal resolution if it is ImageHeight/2 pixels high and ImageWidth/2 pixels wide.

In the preferred embodiment, the VectorSet field is a number from 0 to 7 which is used to select one of eight vector sets to use for decoding an image. Each of the vector sets contains 128 ordered pairs which may be thought of as points defined by X and Y coordinates. In all the vector sets, the ordered pairs are clustered about the point (128, 128); however, the average distance between the ordered pairs and the center point (128, 128) varies among the vectors sets. In VectorSet 0, the ordered pairs are closely clustered about (128, 128). VectorSet 0 thus corresponds to the lowest quantization level. As one moves from VectorSet 0 to VectorSet 7, the ordered pairs cluster less closely around (128, 128). VectorSet 7 thus corresponds to the highest quantization level.

The eight vector sets used in the preferred embodiment of the present invention are attached hereto as Appendix I. In the preferred embodiment, the vectors have arithmetic values in the range -128 to 127. The vector values shown in Appendix I have 128 added to them, so that they are in the range 0 to 255. Other vector sets may be used without departing from the spirit of the present invention.

In the preferred embodiment, the value of the VectorSet field may vary from image to image, thus allowing the encoder to vary the quantization level between images. In this embodiment, the vector set selected by the VectorSet field is used to decode the Y component image. The vector set selected by the value VectorSet/2 is used for the U, V components. Better quantization is normally required for encoding the U, V component images, since these components are typically subsampled spatially. In an alternate embodiment, a single vector set may be used to encode and decode all images in a sequence.

A Huffman table descriptor for each image is also included in the format of bitstream 100. The Huffman table is preferably of the form shown in Table I below:

                  TABLE I                                                          ______________________________________                                                    0[xx . . . x]                                                                  10[xx . . . x]                                                                 110[xx . . . x]                                                                1110[xx . . . x]                                                               11110[xx . . . x]                                                              111110[xx . . . x]                                                             1111110[xx . . . x]                                                            11111110[xx . . . x]                                                ______________________________________                                    

Byte K in the Huffman table descriptor indicates how many "x bits" there are in row K of the above table. The Huffman decoding operation collects bits from the bitstream one at a time until a code-word in a code-book is recognized. Huffman decoder returns a code-book index representing the position of a code-word within a lexicographically-ordered list of code-words.

Following the above header information in bitstream 100 is the Huffman encoded data describing the Y plane. Data for the V and U planes immediately follows the Y plane data. In the preferred embodiment, the V and U data describe a bitmap which is 1/4 the size horizontally and 1/4 the size vertically of the Y bitmap. The final result is a YUV 4:1:1 image which may be displayed directly by a display processor, or converted to some other display format if desired.

Decoding Procedure

The decoding procedure for a still image can be described by the c-language pseudo code in Table II below. In the pseudo code, the function huffdec() performs a huffman decode operation as described above and returns an unsigned integer representing the code-book index:

                  TABLE II                                                         ______________________________________                                         Define                                                                         Width = ImageWidth, divided by 2 depending on the                              value of Flags                                                                 Height = ImageHeight, divided by 2 depending on                                the value of Flags                                                             Then:                                                                          unsigned char *curr,*prev;                                                     unsigned int *vec;                                                             for (x=0; x<Width; x++) // Fill first line with 128's                          bitmap[0] [x]= 128;                                                            for (y=0; y<Height; y++) // for each line of image                             //  point to beginning of current line and previous line                       curr = &bitmap[y] [0];                                                         prev = &bitmap[y - (y -= 0)] [0];                                              for (x=0; x<Width; x+=2) // for each pair of                                                 pixels                                                           {                                                                              k = index[huffdec() ];                                                         // Now do either a run-length of O's or a single vector,                       // depending on the value of k.                                                if (k > 256) // run-length of O's?                                             {                                                                                     for (i=0; i<k-256; i++)                                                            *curr++ = *prev++;                                                         x += k-258;                                                             }                                                                              else // apply a single vector                                                  {                                                                                     vec = vectors + k;                                                             *curr++ = clamp (*prevv++ + *vecc++);                                          *curr++ = clamp (*prevv++ + *vecc++);                                   }                                                                              }                                                                              }                                                                              where:                                                                         'vectors' is a pointer to the vector set to use for                            this image, and index[] is the following array:                                index[] = {                                                                     2,   4,    258,    6,   8,  260,  10,  12,                                     262,                                                                               264,    14,    16, 266,  18,  20,  22,                                     24,  26,    28,   268,  30,  32, 270, 272,                                     34,  36,    38,    40, 274,  42,  44, 276,                                     46,  48,   278,    50,  52, 280,  54,  56,                                     282,                                                                                58,    60,   284,  62,  64, 286,  66,                                     68, 288,    70,    72,  74,  76,  78,   80,                                    82,  84,    86,    88,  90,  92,  94,  96,                                     98, 100,   102,   104, 106, 108, 110, 112,                                    114, 116,   118,   120, 122, 124, 126, 128,                                    130, 132,   134,   136, 138, 140, 142, 144,                                    146, 148,   150,   152, 154, 156, 158, 160,                                    162, 164,   166,   168, 170, 172, 174, 176,                                    178, 180,   182,   184, 186, 188, 190, 192,                                    194, 196,   198,   200, 202, 204, 206, 208,                                    210, 212,   214,   216, 218, 220, 222, 224,                                    226, 228,   230,   232, 234, 236, 238, 240,                                    242, 244,   246,   248, 250, 252, 254,                                         }                                                                              and clamp(x) is a function defined as follows:                                 clamp(x) = 0 if x<l28                                                          x-128 if 128 >= x < 384                                                        255 if x >= 384                                                                ______________________________________                                    

After executing the above procedure, the decoder then scales the image up horizontally and/or vertically by a factor of two, if requested to by the Flags field in the header.

The decoding procedure for an inter-frame (non-still) image is similar to that described in Table II, and is obtained by deleting the first 2 lines of code, and changing the line

    prev=&bitmap[y-(y!=0)][0];

    to

    prev=&prev.sub.-- bitmap[y][0];

Encoding Procedure

Referring now to FIG. 3, there is shown an overall flow diagram for encoding an image in real time according to a preferred embodiment of the present invention.

The first step is to determine whether to encode the image as an intra-frame (a still image) or an inter-frame (an image encoded relative to the previous image in the sequence). For this purpose, a user parameter called StillPeriod is used. The user sets StillPeriod to a given value (K) to force every Kth image to be encoded as a still (INTRA) image. For efficiency of encoding, an encoder may choose to use an INTRA frame even for images in between every-Kth image. For example, if there is a scene cut or if the video enters a period of very high motion, then an intra-frame image will be more efficient to encode than an inter-frame, because the correlation between adjacent images will be too small to be advantageous.

As shown in FIG. 3, means 310 first computes the absolute difference (ABSDIF) between frame N and the previous frame (N-1). This involves summing the absolute value of the differences between all pixels in the two images. For efficiency of computation it is preferable to only use a subset of the pixels in the two images for the purpose of comparison. This provides as nearly an accurate measure of the difference between the two images at a greatly reduced computational cost. After this computation, means 320 (i) compares the absolute difference between frame N and a previous frame N-1 with a predetermined threshold, and (ii) computes the value of N mod StillPeriod. If means 320 determines (i) that the absolute difference is greater than the predetermined threshold or (ii) that (N mod StillPeriod) is zero, then the frame type is set to INTRA by means 325. Otherwise, the frame type is set to INTER by means 330. In alternate embodiments, parameters other than the absolute difference between all pixels in frames N and N-1 may be used in determining how to set the frame type. For example, the mean-square error between pixels in frames N and N-1 or the relative difference between such pixels may be used.

After determining whether to encode as an INTRA or INTER image, means 340a next computes the pixel differences which are to be encoded. As described in the discussions of FIGS. 2A, 2B above, if the image is an INTRA, each pixel has subtracted from it the value of the pixel immediately above it in the same image. (For the top row, a "phantom value" of 128 is used for these pixels.) If the image is an INTER image, each pixel has subtracted from it the value of the pixel in the same spatial location in the previous image. The pixel differences are then vector-quantized and run-length encoded by means 340b . Further details of this vector-quantization and run-length encoding procedure are shown in FIG. 3A and will be described below. The output of means 340b is a string of bytes with values corresponding to the values in the index[] array (divided by 2). This string of bytes is Huffman encoded by means 360 into variable-length codes. Further details of Huffman encoder 360 are shown in FIG. 3B and will be described below. In the final encoding step, means 380 prepends the proper bitstream header.

Referring now to FIG. 3A, there is shown a flow diagram illustrating the operation of means 340 of FIG. 3. In particular, FIG. 3A shows the run-length encoding and vector quantization procedures of means 340b . The operation of means 340 is performed with a 2-state machine. The two states are denoted as ZERO and NONZERO. The ZERO state indicates that the system is in the middle of processing a run of 0 values. The NONZERO state indicates that non-zero values are being processed. The purpose of the two states is to allow for efficient encoding of consecutive zero differences.

In the first step of FIG. 3A, means 342 initializes the state machine to the NONZERO state. Next, means 344 computes the next pair of pixel differences. In the preferred embodiment, the image is processed in normal raster-scan order, from top to bottom and left to right within each line. The "next pair" of pixels means the next two pixels on the current scan line being processed. As stated above, the differences are taken with the pixels immediately above these pixels (if this image is being encoded as an INTRA) or with the pixels in the same spatial location in the previous image (if this image is being encoded as an INTER image). Since these two values represent pixel differences, they will typically be small, or close to zero.

In the next step, means 346 operates to 2-D vector-quantize the two pixel difference values into a single number (index) between 0 and 127. The possible index values correspond to 128 points in 2-D space known as a "vector set". Geometrically, a vector set represents 128 points in the 2-D square bounded by the values -255 and 255 which have been chosen as reasonable approximations to every point in the square. Thus, if the two pixel difference values are denoted by d1 and d2, they can be represented as a point in the 2-D square with coordinates (d1, d2). The vector quantization operation attempts to choose the closest (in Euclidean distance) of the 128 representative points to be used to encode the point (d1, d2). Since the vector set is relatively small, this choosing operation can be done quickly using a lookup table. According to this procedure, the values d1 and d2 are first limited to the range -127 to +127. Then, the quantity 128 is added to produce values in the range 0 to 255. Next, a value p is calculated according to equation (1) below, wherein "|" represents a bitwise inclusive OR operation, and "<<" and ">>" indicate left and right shift operations, respectively, of the left operand by the number of bit positions given by the right operand:

    p=(d1>>2)|(d2 >>2<<6)                             (1)

The value of p is in the range 0 to 4095. The value at position `p` in a 4096-entry lookup table is then used to get the index corresponding to the closest representative point in the vector set corresponding to (d1, d2). Although a slight inaccuracy in the computation is introduced by not using the lower 2 bits of d1 and d2, without this step the lookup table would be 64K instead of 4K. A separate lookup table is required for each of the eight vector sets for a total size of 32K bytes. During encoding, the degree of quantization used (e.g., the VectorSet value chosen) is varied by known feedback processes which monitor the size of encoded images in the bitstream.

The remainder of FIG. 3A maintains the value of a variable `run` which indicates how many consecutive index values of 0 have been produced. When a run of 0 values is ended, means 350 outputs the value 128+run. For each non-zero index, means 354 outputs the index value itself. Means 358 functions to repeat the process (starting from means 344) until all pixels have been processed. The encoding procedure shown in FIGS. 3, 3A for a still (INTRA) image can be described by the c-language pseudo code in Table III below:

                  TABLE III                                                        ______________________________________                                         Define                                                                         Width = ImageWidth, divided by 2 depending on the                                          value of Flags                                                     Height = ImageHeight, divided by 2 depending on the                                        value of Flags                                                     Then                                                                           unsigned char *curr, *prev,grey[XMAX];                                         unsigned char *lookup                                                          for (x=0; x<Width; x++) // make a line of 128's                                grey[x] = 128;                                                                 state = NONZERO;                                                               for (y=0; y<Height; y++) // for each line of image                             curr = &bitmap[y] [0];                                                         if (y > 0)                                                                             prev = &bitmap[y-1] [0];                                               else                                                                           prev = &grey[0];                                                               for (x=0; x<Width; x+=2)                                                       {                                                                                      *prev++ + 128);r++                                                             *prev++ + 128);r++                                                             index = lookup[ (dl >> 2) | (d2 >> 2 << 6) ];                         if (state = = ZERO)                                                            {                                                                                if (index == 0)                                                                  run++;                                                                       else                                                                             { huffenc(run + 128); huffenc(index);                                            state = NONZERO; }                                                       }                                                                              else if (state == NONZERO)                                                     {                                                                                if (index == 0)                                                                  { run = 1; state = ZERO; }                                                   else                                                                             huffenc(index);                                                            }                                                                      }                                                                              }                                                                              where                                                                          'lookup' is a pointer to the 4K difference-pair-to-                            vector-index lookup table for the current vector set;                          huffenc(x) is a function to output the appropriate                             Huffman codeword such that index[huffdec(huffenc(x))]                          = x.                                                                           ______________________________________                                    

The encoding procedure for an inter-frame image is similar to that described in Table III, and is obtained by deleting the first 2 lines of code, and changing the lines

    ______________________________________                                         if (y > 0)                                                                     prev = &bitmap[y-1] [0];                                                       else                                                                           prev = &grey[0];                                                               to                                                                             prev = &prev.sub.-- bitmap[y] [0];                                             ______________________________________                                    

Referring now to FIG. 3B, there is shown a flow diagram illustrating the Huffman encoding of the byte values output by means 340b . The Huffman encoding step replaces the fixed 8-bit codes with a statistically-optimized set of variablelength codes. Before the Huffman encoding begins, two tables (table1 and table2) are precalculated to specify, for each 8-bit value to be Huffman encoded, the number of bits in the Huffman code and the actual bits themselves. The bits are top-justified in a 16-bit value. The Huffman encoding operation is assisted by a 16-bit register called `bitbuf` in which bits are collected. Another register, `rbits` is used to indicate how many unused bits there are remaining in `bitbuf`. Means 361 initially sets rbits to 16, since `bitbuf` is initially empty.

Means 362 reads the next byte of data and looks up `numbits` and `bits` in the two tables. Decision block 363 determines whether there is room enough in `bitbuf` to hold the entire Huffman code word, i.e., is numbits <=rbits? If so, then `bits` is ORed into `bitbuf` by means 364, and `rbits` is reduced by the value of `numbits` by means 365. If it is determined by decision block 363 that the bits do not fit in `bitbuf` then the encoder puts as many bits as will fit into `bitbuf`, outputs `bitbuf` puts the remaining bits into bitbuf, and sets rbits=16-diff. More particularly, means 366 determines a value diff by subtracting rbits from numbits. Means 367 puts as many bits as will fit into bitbuf by ORing (bitbuf>>rbits) with (bits<<diff). Means 368 then outputs bitbuf and means 369 sets bitbuf to bits and rbits to 16 minus diff.

Decision block 370 determines whether the processing of all bytes is completed. If it is determined that all bytes have not been processed, the above process (starting with means 362) is repeated.

SYSTEM DETAILS

Two overall system block diagrams are shown in FIGS. 4A, 4B. FIG. 4A shows a block diagram for recording and FIG. 4B shows a block diagram for playback; however, the same system can be used (even simultaneously) for either recording (encoding) or playback (decoding).

Referring now to FIG. 4A, the analog video is first digitized by video digitizer 410, and the digital images are stored in memory 420 in "YUV-9" format. This format consists of three planes of 8-bit pixels: one Y plane, one U plane, and one V plane. The U and V planes are stored at 1/4 the resolution in each dimension compared to the Y plane. Means 430 includes a set of control and synchronization routines which examine the images and invoke encoder 440 in order to compress successive frames of the digitized video. The bitstreams are then output to memory, from which they can be stored to hard disk or sent over a network.

Referring now to FIG. 4B, a playback system according to the present invention is shown. The playback diagram of FIG. 4B is the inverse of the record diagram shown in 4A. Thus, means 470 accepts as input compressed data and invokes decoder 480 as appropriate to decompress successive frames of the video. The decompressed video is stored in memory 460 in YUV-9 format. Display hardware 450 produces analog video from the YUV-9 data.

In the preferred embodiment, digitizer 410 can be programmed to digitize horizontally or vertically at any resolution. In effect, this means that the digitizer can be used to do part of the compression process. By programming the digitizer to a lower resolution, there will be less data for the encoder to compress and the final data size will be smaller.

In addition, digitizer 410 may dynamically alter the digitizer resolution (either horizontally or vertically) when the video becomes "hard" to compress. A method and apparatus for dynamically altering resolution based on image complexity is implemented in U.S. patent application entitled, "Method and Apparatus For Encoding Selected Images At Lower Resolution" by A. Alattar, S. Golin and M. Keith, filed Mar. 25, 1992, assigned Ser. No. 07/856,515, which application is assigned to the assignee of the present application and the contents of which are hereby incorporated herein by reference.

In the real time video system described above, the encoder takes incoming digitized images, compresses them, and outputs the compressed bitstream to a buffer in memory for extraction by the application. The simplistic view of the system assumes that everything works "ideally", so that a new compressed frame is generated exactly F times per second, where F is the desired frame rate requested by the user. However, there are at least two conditions which typically occur to make the operation of the system less than ideal:

(1) The analog video source may disappear for a period, thus precluding new digitized images from being obtained by the digitizer; and

(2) The application may not extract compressed frames from the buffer fast enough, which means that the encoding system gets "stalled" by the inability to output more compressed frames (caused by the output buffer being full).

In either case, if the encoder simply fails to output frames, this will result in a loss of time synchronization. For example, if the system is encoding at 30 frames per second, the playback system would expect to get 900 frames in 30 seconds. If, due to conditions (1) or (2), less than 900 frames are generated (for example, 840), then upon playback the playback system will play these 840 frames at 30 frames per second, and the playback of these frames will occupy only 28 seconds. This is not acceptable, since the video information upon playback will not occupy the same amount of real time that it did during recording. This will be evident to the viewer by, for example, loss of audio/video synchronization.

A solution to this problem is presented by what will be termed "sync frames". During encoding, means 430 keeps track of real time using a clock signal. It attempts to generate F compressed data frames per second, as requested by the user, and it monitors how well it is doing. If at any point it determines that it is behind (i.e., fewer frames have been generated so far than there should be), it inserts a "sync frame" into the compressed buffer. A "sync frame" is a compressed data frame that appears in the bitstream just like a normal compressed frame (and so travels through the record and playback systems without any special handling) but which can be detected by the playback process as special.

The sync frame consists of the bitstream header (described above) with the DataSize field set to 128 and the other fields set to the appropriate values. A sync frame in effect counts the passage of time without causing a new image to appear on the screen. When the decoder encounters a sync frame, it simply copies the previous image to the current image bitmap. This results in no change to the display but the proper passage of time, so that accurate time synchronization results. Thus, if a system bottleneck occurs so that only 840 "real" compressed frames are created during a 30-second period, then means 430 will insert 60 sync frames. Thus, over the 30-second period there will be exactly 900 frames, as desired, but 60 of them will be sync frames. On playback, there will be some visual anomalies when the sync frames are processed, but exact time synchronization will be maintained.

The present invention may be implemented in real time (both compression and decompression) using an Intel model i750PB processor. Other processors, including Intel 386/486 processors, may be used to scalably decode video data which has been encoded accorded to the present invention.

Scalability

By manipulating the Flags and StillPeriod parameters in the encoder, it is possible to create real time video files which can be scalably decoded; that is, yielding reasonable-quality playback on a typical micro-processor (for example, a 386/486 class processor), and better quality on a higher-performance video signal processor chip (for example, a DVI i750 chip). In a preferred embodiment, real time video files which can be scalably decoded are created by setting Flags so that half vertical and half horizontal resolution is selected. The decoder would therefore normally be expected to scale up the image by 2× both vertically and horizontally after decoding. According to the present invention, if a sequence of 256×240 images is compressed at 128×120 resolution, it can be decompressed and displayed as a sequence of 128×120 images on a typical micro-processor. By opting not to interpolate the 128×120 images back up to 256×240 images, a typical micro-processor can be used to reproduce image sequences encoded in real time with a reasonable degree of quality. The image quality level can be improved through the use of a higher-performance video signal processor which reproduces the sequence by decoding and then interpolating back up to 256×240 images. Thus, the same encoded sequence can be reproduced at different quality levels depending on the limitations of the decompression hardware. Another aspect of real time video files which can be scalably decoded would allow a typical micro-processor system to use a VGA for display whereas a video signal processor system may use a 24-bit-color i750DB display. The micro-processor system might choose to display in monochrome to avoid messy YUV-to-VGA-clut conversion.

Referring now to FIG. 5A, there is shown a flow diagram illustrating the operation of a preferred system for scalably decoding a digital video sequence in accordance with the present invention. Means 510 are provided for identifying the processor to be used for decoding a sequence of encoded digital video images. Since a variety of different processors having different levels of processing capabilities may be used to decode the encoded sequence, means 510 will return a value representative of the processing power of the processor to be used for decoding. In a preferred embodiment, the value returned by means 510 will indicate whether the decoding processor is (i) a 386 class processor, (ii) a 486 class processor, or (iii) an i750PB processor. In alternative embodiments, means 510 may return values representative of the processor power of other processors and other processor classes. Means 512 is provided for determining a pixel depth interpolation level in accordance with the processing power identified by means 510. The pixel depth interpolation level determined by means 512 represents the pixel depth (in bits) of the decoded images to be produced by the scalable decoder. Means 514 are provided for selecting an encoded frame from a sequence of encoded digital frames. Means 516 is provided for comparing the pixel depth interpolation level with at least one threshold. If the pixel depth interpolation level is greater than a predetermined threshold, then means 518 forms a scalably decoded frame by interpolating the selected frame in accordance with a first pixel depth level. If the pixel depth interpolation level is not greater than the predetermined threshold, then means 520 forms a scalably decoded frame by interpolating the selected frame in accordance with a second pixel depth level, wherein the second pixel depth level has fewer bits than the first pixel depth level. In the preferred embodiment, the first pixel depth level is 24 bits and means 518 forms decoded frames having 24-bit pixels, while the second pixel depth level is 8 bits and means 520 forms decoded frames having 8-bit pixels. In alternative embodiments, decoded images of different pixel depths may be formed by means 518, 520. In still further embodiments, means 516 may compare the pixel depth interpolation level returned by means 512 with more than one threshold and chose between producing decoded images having more than two possible pixel depth levels. Means 522 is provided for determining whether each frame in the sequence of encoded frames has been decoded. If the end of the sequence has not been reached, the process is repeated from means 514.

In accordance with another aspect of scalable decoding shown in FIG. 5B, the present invention may be used to periodically replace one or more frames in an encoded sequence of frames with a corresponding number of sync frames. In this embodiment, sync frames are used to replace encoded frames that have been encoded using relative encoding and which will not be needed to decode subsequent frames in the sequence. For example, in a sequence of frames that have been encoded such that every tenth frame is encoded as a still frame, with the remaining frames being encoded using relative encoding, one or more frames immediately preceding each still frame may be replaced with a sync frame. The number of sync frames used will vary depending on the processing power of the processor being used for decoding.

Referring now to FIG. 5B, there is shown a flow diagram illustrating the system for scalable decoding described in the paragraph immediately above. Means 530 is provided for identifying the processor to be used for decoding a sequence of encoded digital video images. Means 530 functions substantially in accordance with means 510 described above. Means 532 is provided for determining a frame discard interval value in accordance with the processing power identified by means 530. The frame discard interval value is used to identify positions in the encoded sequence where frames that have been encoded using relative encoding will be "discarded" and replaced with sync frames. Means 534 is provided for initializing a counter, means 536 is provided for selecting an encoded frame from a sequence of encoded digital frames, means 538 is provided for decoding the encoded frame selected by means 536, and means 540 is provided for incrementing the counter. Means 542 compares the counter to the frame discard interval value. If the counter is larger than the frame discard interval value, then means 544 selects a subsequent frame from the sequence of encoded frames and means 546 determines whether the subsequent frame was encoded using relative encoding. If relative encoding was used to encode the selected subsequent frame, then the selected subsequent frame is "discarded" by means 548 and replaced with a sync frame which is then decoded. In the preferred embodiment, the present invention continues replacing subsequent selected frames with sync frames until it reaches a frame that was not encoded using relative encoding. Means 550 is provided for determining whether the end of the sequence has been reached. If the end of the sequence has not been reached, means 552 initializes the counter and the process is repeated from means 538.

In a still further aspect of scalability illustrated in FIG. 5C, during real time compression a user may set the parameter "StillPeriod" to P, thus requiring every Pth image to be encoded as a still. P may also be set automatically by the encoding system, based on the processing power of the expected decoding hardware (the target decoder). The other images may then be encoded using inter-frame differences (relative encoding). P can typically be set quite small without adversely affecting image quality. By compressing a sequence with P=3, the processing requirements for a micro-processor type system can be reduced without adversely affecting image quality. For example, decompressing and displaying still images using a 386/486 processor typically yields a 10 fps display. This frame rate can be increased smoothly from 10 fps to 30 fps if P=3.

Referring now to FIG. 5C, there is shown a flow diagram illustrating the system for scalable real time encoding described in the paragraph immediately above. Means 560 is provided for selecting a target processor class to be used for decoding a scalably encoded sequence of digital video frames. Since a variety of different processors having different levels of processing capabilities may be used to decode the encoded sequence, means 560 will return a value representative of the processing power of the processor that the system (or its user) anticipates will be used for decoding. In a preferred embodiment, the value returned by means 560 will indicate whether the target decoding processor is (i) a 386 class processor, (ii) a 486 class processor or (iii) an i750PB processor. In alternative embodiments, means 560 may return values representative of other processors or other processor classes. Means 562 is provided for setting the StillPeriod value P in accordance with the value returned by means 560. Means 564 is provided for initializing a counter and means 566 is provided for selecting a frame for encoding. Means 568 compares the counter to a predetermined constant. If the counter is equal to the constant, means 570 encodes the selected frame as a still frame. otherwise, means 572 encodes the selected frame using relative encoding. Means 574 then increments the counter and means 576 compares the counter to P. If the counter is less than or equal to P, the process is repeated from means 566, otherwise, the process is repeated from means 564.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes of the invention. Accordingly, reference should be made to the appended claims, rather than the foregoing specification, as indicating the scope of the invention.

    ______________________________________                                         APPENDIX                                                                       ______________________________________                                         Vector set 0                                                                   128   128     132     132   124   124   127   133                              129   123     133     127   123   129   140   140                              116   116     131     141   125   115   141   131                              115   125     119     137   137   119   137   119                              119   137     140     149   116   107   149   140                              107   116     124     144   132   112   144   124                              112   132     150     150   106   106   130   152                              126   104     152     130   104   126   151   162                              105   94      162     151   94    105   162   162                              94    94      139     163   117   93    163   139                              93    117     113     149   143   107   149   113                              107   143     120     157   136   99    157   120                              99    136     127     167   129   89    167   127                              89    129     164     177   92    79    177   164                              79    92      150     177   106   79    177   150                              79    106     178     178   78    78    101   155                              155   101     155     101   101   155   137   180                              119   76      180     137   76    119   106   163                              150   93      163     106   93    150   115   172                              141   84      172     115   84    141   180   195                              76    61      195     180   61    76    164   195                              92    61      195     164   61    92    196   196                              60    60      150     198   106   58    198   150                              58    106     124     186   132   70    186   124                              70    132     91      171   165   85    171   91                               85    165     99      180   157   76    180   99                               76    157     134     202   122   54    202   134                              54    122     182     215   74    41    215   182                              41    74      200     215   56    41    215   200                              41    56      164     216   92    40    216   164                              40    92      108     193   148   63    193   108                              63    148     217     217   39    39    128   128                              Vector set 1                                                                   128   128     133     133   123   123   126   135                              130   121     135     126   121   130   143   143                              113   113     132     143   124   113   143   132                              113   124     117     139   139   117   139   117                              117   139     142     154   114   102   154   142                              102   114     123     147   133   109   147   123                              109   133     155     155   101   101   130   157                              126   99      157     130   99    126   155   168                              101   88      168     155   88    101   169   169                              87    87      141     170   115   86    170   141                              86    115     110     153   146   103   153   110                              103   146     118     162   138   94    162   118                              94    138     127     175   129   81    175   127                              81    129     171     186   85    70    186   171                              70    85      154     187   102   69    187   154                              69    102     187     187   69    69    96    160                              160   96      160     96    96    160   139   190                              117   66      190     139   66    117   102   170                              154   86      170     102   86    154   112   181                              144   75      181     112   75    144   190   207                              66    49      207     190   49    66    171   208                              85    48      208     171   48    85    209   209                              47    47      154     211   102   45    211   154                              45    102     123     197   133   59    197   123                              59    133     84      180   172   76    180   84                               76    172     94      190   162   66    190   94                               66    162     135     216   121   40    216   135                              40    121     192     232   64    24    232   192                              24    64      213     232   43    24    232   213                              24    43      171     233   85    23    233   171                              23    85      104     205   152   51    205   104                              51    152     234     234   22    22    128   128                              Vector set 2                                                                   128   128     134     134   122   122   126   136                              130   120     136     126   120   130   146   146                              110   110     133     146   123   110   146   133                              110   123     115     141   141   115   141   115                              115   141     145     158   111   98    158   145                              98    111     121     151   135   105   151   121                              105   135     160     160   96    96    131   162                              125   94      162     131   94    125   160   176                              96    80      176     160   80    96    177   177                              79    79      143     178   113   78    178   143                              78    113     107     158   149   98    158   107                              98    149     116     169   140   87    169   116                              87    140     127     184   129   72    184   127                              72    129     180     197   76    59    197   180                              59    76      159     198   97    58    198   159                              58    97      198     198   58    58    89    167                              167   89      167     89    89    167   141   202                              115   54      202     141   54    115   97    178                              159   78      178     97    78    159   109   191                              147   65      191     109   65    147   202   223                              54    33      223     202   33    54    179   223                              77    33      223     179   33    77    225   225                              31    31      159     227   97    29    227   159                              29    97      122     211   134   45    211   122                              45    134     76      190   180   66    190   76                               66    180     87      202   169   54    202   87                               54    169     136     233   120   23    233   136                              23    120     204     251   52    5     251   204                              5     52      230     251   26    5     251   230                              5     26      180     253   76    3     253   180                              3     76      99      220   157   36    220   99                               36    157     254     254   2     2     128   128                              Vector set 3                                                                   128   128     135     135   121   121   126   137                              130   119     137     126   119   130   149   149                              107   107     134     150   122   106   150   134                              106   122     112     144   144   112   144   112                              112   144     148     164   108   92    164   148                              92    108     120     155   136   101   155   120                              101   136     166     166   90    90    131   169                              125   87      169     131   87    125   166   185                              90    71      185     166   71    90    186   186                              70    70      146     188   110   68    188   146                              68    110     103     163   153   93    163   103                              93    153     114     176   142   80    176   114                              80    142     127     195   129   61    195   127                              61    129     190     210   66    46    210   190                              46    66      165     212   91    44    212   165                              44    91      212     212   44    44    82    174                              174   82      174     82    82    174   143   216                              113   40      216     143   40    113   91    187                              165   69      187     91    69    165   105   203                              151   53      203     105   53    151   216   240                              40    16      240     216   16    40    189   241                              67    15      241     189   15    67    243   243                              13    13      165     246   91    10    246   165                              10    91      120     226   136   30    226   120                              30    136     66      201   190   55    201   66                               55    190     79      216   177   40    216   79                               40    177     138     253   118   3     253   138                              3     118     219     255   37    1     255   219                              1     37      249     255   7     1     255   249                              1     7       190     255   66    1     255   190                              1     66      94      237   162   19    237   94                               19    162     255     255   1     1     128   128                              Vector set 4                                                                   128   128     136     136   120   120   126   138                              130   118     138     126   118   130   152   152                              104   104     134     154   122   102   154   134                              102   122     110     146   146   110   146   110                              110   146     152     170   104   86    170   152                              86    104     120     160   136   96    160   120                              96    136     172     172   84    84    132   176                              124   80      176     132   80    124   174   196                              82    60      196     174   60    82    196   196                              60    60      150     198   106   58    198   150                              58    106     98      170   158   86    170   98                               86    158     112     186   144   70    186   112                              70    144     126     206   130   50    206   126                              50    130     200     226   56    30    226   200                              30    56      172     226   84    30    226   172                              30    84      228     228   28    28    74    182                              182   74      182     74    74    182   146   232                              110   24      232     146   24    110   84    198                              172   58      198     84    58    172   102   216                              154   40      216     102   40    154   232   255                              24    0       255     232   0     24    200   255                              56    0       255     200   0     56    255   255                              0     0       172     255   84    0     255   172                              0     84      120     244   136   12    244   120                              12    136     54      214   202   42    214   54                               42    202     70      232   186   24    232   70                               24    186     140     255   116   0     255   140                              0     116     236     255   20    0     255   236                              0     20      255     255   0     0     255   255                              0     0       200     255   56    0     255   200                              0     56      88      255   168   0     255   88                               0     168     255     255   0     0     128   128                              Vector set 5                                                                   128   128     138     138   118   118   124   142                              132   114     142     124   114   132   158   158                              98    98      136     158   120   98    158   136                              98    120     106     150   150   106   150   106                              106   150     156     180   100   76    180   156                              76    100     118     166   138   90    166   118                              90    138     182     182   74    74    132   186                              124   70      186     132   70    124   182   208                              74    48      208     182   48    74    210   210                              46    46      154     212   102   44    212   154                              44    102     92      178   164   78    178   92                               78    164     108     196   148   60    196   108                              60    148     126     222   130   34    222   126                              34    130     214     244   42    12    244   214                              12    42      180     246   76    10    246   180                              10    76      246     246   10    10    64    192                              192   64      192     64    64    192   150   252                              106   4       252     150   4     106   76    212                              180   44      212     76    44    180   96    234                              160   22      234     96    22    160   252   255                              4     0       255     252   0     4     214   255                              42    0       255     214   0     42    255   255                              0     0       180     255   76    0     255   180                              0     76      118     255   138   0     255   118                              0     138     40      232   216   24    232   40                               24    216     60      252   196   4     252   60                               4     196     142     255   114   0     255   142                              0     114     255     255   0     0     255   255                              0     0       255     255   0     0     255   255                              0     0       214     255   42    0     255   214                              0     42      80      255   176   0     255   80                               0     176     255     255   0     0     128   128                              Vector set 6                                                                   128   128     140     140   116   116   124   144                              132   112     144     124   112   132   164   164                              92    92      138     164   118   92    164   138                              92    118     102     154   154   102   154   102                              102   154     162     188   94    68    188   162                              68    94      114     174   142   82    174   114                              82    142     192     192   64    64    134   196                              122   60      196     134   60    122   192   224                              64    32      224     192   32    64    226   226                              30    30      158     228   98    28    228   158                              28    98      86      188   170   68    188   86                               68    170     104     210   152   46    210   104                              46    152     126     240   130   16    240   126                              16    130     232     255   24    0     255   232                              0     24      190     255   66    0     255   190                              0     66      255     255   0     0     50    206                              206   50      206     50    50    206   154   255                              102   0       255     154   0     102   66    228                              190   28      228     66    28    190   90    254                              166   2       254     90    2     166   255   255                              0     0       255     255   0     0     230   255                              26    0       255     230   0     26    255   255                              0     0       190     255   66    0     255   190                              0     66      116     255   140   0     255   116                              0     140     24      252   232   4     252   24                               4     232     46      255   210   0     255   46                               0     210     144     255   112   0     255   144                              0     112     255     255   0     0     255   255                              0     0       255     255   0     0     255   255                              0     0       232     255   24    0     255   232                              0     24      70      255   186   0     255   70                               0     186     255     255   0     0     128   128                              Vector set 7                                                                   128   128     142     142   114   114   124   146                              132   110     146     124   110   132   170   170                              86    86      140     172   116   84    172   140                              84    116     96      160   160   96    160   96                               96    160     168     200   88    56    200   168                              56    88      112     182   144   74    182   112                              74    144     204     204   52    52    134   210                              122   46      210     134   46    122   204   242                              52    14      242     204   14    52    244   244                              12    12      164     248   92    8     248   164                              8     92      78      198   178   58    198   78                               58    178     100     224   156   32    224   100                              32    156     126     255   130   0     255   126                              0     130     252     255   4     0     255   252                              0     4       202     255   54    0     255   202                              0     54      255     255   0     0     36    220                              220   36      220     36    36    220   158   255                              98    0       255     158   0     98    54    246                              202   10      246     54    10    202   82    255                              174   0       255     82    0     174   255   255                              0     0       255     255   0     0     250   255                              6     0       255     250   0     6     255   255                              0     0       202     255   54    0     255   202                              0     54      112     255   144   0     255   112                              0     144     4       255   252   0     255   4                                0     252     30      255   226   0     255   30                               0     226     148     255   108   0     255   148                              0     108     255     255   0     0     255   255                              0     0       255     255   0     0     255   255                              0     0       252     255   4     0     255   252                              0     4       60      255   196   0     255   60                               0     196     255     255   0     0     128   128                              ______________________________________                                     

What is claimed is:
 1. A computer implemented method for scalably encoding in real time a sequence of digital video frames, comprising the steps of:(A) selecting a target digital processor class to be used for decoding said sequence; (B) selecting a stillperiod value in accordance with the selected target digital processor class; and (C) encoding every Nth frame of said sequence as a still frame and encoding all other frames using interframe difference values, wherein N is equal to said stillperiod value.
 2. An apparatus for scalably encoding in real time a sequence of digital video frames, comprising:(A) means for selecting a target digital processor class to be used for decoding said sequence; (B) means for selecting a stillperiod value in accordance with the selected target digital processor class; and (C) means for encoding every Nth frame of said sequence as a still frame and encoding all other frames using interframe difference values, wherein N is equal to said stillperiod value. 